Scallop-resistant track link and method of making same

ABSTRACT

A track link includes an elongate link body formed of a link body material that varies in hardness to form a first lower hardness zone, a second lower hardness zone, and a higher hardness zone. The higher hardness zone includes an upper rail surface of the elongate link body and extends substantially throughout the elongate link body outside of the first and second lower hardness zones, which surround the track pin bores. Related methodology for making a track link is also disclosed.

TECHNICAL FIELD

The present disclosure relates generally to track systems inground-engaging machines and, more particularly, to a track link havingdifferent hardness zones to retard scallop formation.

BACKGROUND

A wide variety of machines utilize tracks as ground-engaging propulsionelements, notably track-type tractors. It is common for such tracks toinclude a plurality of rotatable track-contacting elements, with each oftwo tracks at opposite sides of the machine forming endless loops movedabout rotating components during operation. Such tracks typically eachinclude two chains of coupled-together links, with bolted-on trackshoes, and the links held together by way of track pins. The demandsplaced upon such machines and their associated tracks can be quitesubstantial with extremely high torsional loads, shearing loads,impacts, and still others. Machine tracks are commonly fairly robust toprovide an operating life of hundreds, or even more preferablythousands, of hours despite significant stresses, strain, and wear.

In recent years, understanding and addressing wear phenomena in machinetracks has received considerable engineering attention. The various wearphenomena and wear rates experienced by machine track are typically aresult of how the machine is used, the skill and experience of theoperator, as well as the particular underfoot conditions and thesubstrate materials in the operating environment. Machines operated insandy materials, for example, tend to wear certain components relativelymore rapidly than machines used in predominately soil and/or clay, coal,or landfill waste environments. Field service life of machine track canthus vary based upon the factors mentioned above. Since machine trackcomponents can be relatively expensive to service and replace, not tomention the negatives of machine down time, engineering efforts in thisfield have often centered around reducing and managing wear between andamong components. It has become well known, for example, that tracklinks can wear according to a “scalloping” phenomenon where a centerregion of the track line wears relatively. U.S. Pat. No. 3,955,855 toMassieon et al. is directed to a track-type machine having track linkswith contact surfaces formed of relatively high wear-resistant material.The material is apparently a composite alloy metallurgically bonded to abase material of the track links.

SUMMARY OF THE INVENTION

In one aspect, a track for a ground-engaging machine includes a trackchain assembly having a first track chain, and a second track chainextending in parallel with the first track chain, and each of the firsttrack chain and the second track chain being formed by a plurality oftrack links. Each of the plurality of track links forming the firsttrack chain and the plurality of track links forming the second trackchain include an elongate link body having a first link body end with afirst link strap and a second link body end with a second link strap.Each elongate link body includes a lower shoe-mounting surface, anupper-rail surface, a first inner surface defining a first track pinbore adjacent to the first link body end, and a second inner surfacedefining a second track pin bore adjacent to the second link body end.Each elongate link body is formed of a link body material that varies inhardness within the elongate link body to form a first lower hardnesszone, a second lower hardness zone, and a higher hardness zone. Thefirst lower hardness zone includes the first inner surface and extendscircumferentially around the first track pin bore, the second lowerhardness zone includes the second inner surface and extendscircumferentially around the second track pin bore. The higher hardnesszone includes at least a majority of the upper rail surface and extendsthroughout the elongate link body outside of the first lower hardnesszone and the second lower hardness zone.

In another aspect, a track link for a track in a ground-engaging machineincludes an elongate link body having a first link body end and a secondlink body end, a lower shoe-mounting surface, an upper rail surface, afirst inner surface defining a first track pin bore adjacent to thefirst link body end, and a second inner surface defining a second trackpin bore adjacent to the second link body end. The elongate link body isformed of a link body material that varies in hardness within theelongate link body to form a first lower hardness zone, a second lowerhardness zone, and a higher hardness zone. The first lower hardness zoneincludes the first inner surface and extends circumferentially aroundthe first track pin bore. The second lower hardness zone includes thesecond inner surface and extends circumferentially around the secondtrack pin bore. The higher hardness zone includes at least a majority ofthe upper rail surface and extends throughout the elongate link bodyoutside of the first lower hardness zone and the second lower hardnesszone.

In still another aspect, a method of making a track link includes heattreating an elongate link body having a lower shoe-mounting surface andan upper rail surface such that a material extending throughout theelongate link body is hardened. The method further includes temperingthe elongate link body such that a portion of the material that ishardened and forms a first and a second track pin bore in the elongatelink body is softened. The method still further includes forming, by wayof the heat treating and the tempering of the elongate link body, ascallop-retarding pattern of varying hardness where the elongate linkbody has a first lower hardness zone extending circumferentially arounda first track pin bore, a second lower hardness zone extendingcircumferentially around a second track pin bore, and a higher hardnesszone that includes the upper rail surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side diagrammatic view of a machine having a track accordingto one embodiment, and including a detailed enlargement;

FIG. 2 is a sectioned diagrammatic view of a track, in multiple sectionplanes, according to one embodiment;

FIG. 3 is a diagrammatic view of a track link, according to oneembodiment;

FIG. 4 is a diagrammatic view showing stages in making a track link,according to one embodiment;

FIG. 5 is a sectioned side diagrammatic view of a track link in anearlier state of wear, according to one embodiment; and

FIG. 6 is a sectioned side diagrammatic view of a track link in a laterstate of wear, according to one embodiment.

DETAILED DESCRIPTION

Referring to FIG. 1, there is shown a ground-engaging machine 10including a track 12, according to one embodiment. It will be understoodthat track 12 is one track positioned at a first side of a frame 24 ofmachine 10, and a second, substantially identical track would bepositioned at an opposite end of frame 24. Track 12 extends about aplurality of rotatable track-contacting elements. In the illustratedembodiment machine 10 includes a drive sprocket 26, a rear idler 28, anda front idler 30. A track roller frame 22 supports rear idler 28 andfront idler 30. Those skilled in the art will recognize the so-called“high drive” configuration of track 12 in machine 10. It will beappreciated that other track configurations, such as an oval trackconfiguration, would fall within the scope of the present disclosure. Aplurality of additional rotatable track-contacting elements (notnumbered) including track rollers, and carrier rollers, might be part ofthe overall system of which track 12 is a part. Ground-engaging machine10 (hereinafter “machine 10”) is shown in the context of a track-typetractor, but could be any of a variety of other machines such as a trackloader, a half-track machine, or still another piece of off-highwayequipment. FIG. 1 also includes a detailed enlargement illustrating apattern of contact between front idler 30 and a track link 20, wherebysliding contact of parts of idler 30 with track link 20 occurs as track12 advances about front idler 30. This sliding contact has been observedto produce certain forms of wear that can limit track service life,known generally in the art as scalloping. Analogous wear can occur basedupon engagement and disengagement of track 12 with rear idler 28. Aswill be further apparent from the following description, track 12 isuniquely configured to address certain wear phenomena and thus extendedservice life over other known designs.

Referring also now to FIG. 2, track 12 includes a track chain assembly14 having a first track chain 16, and a second track chain 18 extendingin parallel with first track chain 16. Each of first track chain 16 andsecond track chain 18 is formed by a plurality of track links 20. Eachof the plurality of track links 20 forming first track chain 16 and theplurality of track links 20 forming second track chain 18 includes anelongate link body 32. Referring also to FIG. 3, each elongate link body32 has a first link body end 34 with a first link strap 36 and a secondlink body end 38 with a second link strap 40. Each elongate link body32, hereinafter referred to in the singular, includes a lowershoe-mounting surface 42, an upper rail surface 44, a first innersurface 46 defining a first track pin bore 48 and a second inner surface52 defining a second track pin bore 54. First track pin bore 48 has acenter axis 50 and is adjacent to first link body end 34. Second trackpin bore 54 has a center axis 56 and is adjacent to second link body end38. A general construction of track chains 16 and 18 is also shown inFIG. 2, illustrating a track pin 57 that supports a rotating bushing 59and couples together track chains 16 and 18. In other track designsrotating bushings are not used.

In the track drive configuration depicted in FIG. 1, rotation of drivesprocket 26 causes sprocket 26 to engage successively about rotatingbushings associated with each track pin 57, or with track pins directly,causing track 12 to move about the several rotatable track-contactingelements in a forward direction or a reverse direction. Embodiments arecontemplated as shown where each track link 20 in one of track chains 16and 18 is “left handed” while each track link 20 in the other one oftrack chains 16 and 18 is “right handed”, thus the track links 20 in therespective track chain 16 and 18 can be mirror images of one another.Track links 20 can have a serpentine or zigzag configuration.Embodiments are also contemplated where all of track links 20 areidentical in both track chains 16 and 18 and/or neither left handed, orright handed, such as where track links 20 are straight, or still otherconfigurations might be used within the present context. As noted above,track link 20 is uniquely configured to address certain wear phenomena.To this end, elongate link body 32 is formed of a link body material 76that varies in hardness within elongate link body 32 according to ascallop-retarding pattern. The varying hardness forms a first lowerhardness zone 78, a second lower hardness zone 80, and a higher hardnesszone 82. First lower hardness zone 78 includes first inner surface 46and extends circumferentially around first track pin bore 48. Secondlower hardness zone 80 includes second inner surface 52 and extendscircumferentially around second track pin bore 54. Higher hardness zone82 includes at least a majority of upper rail surface 44 and extendsthroughout elongate link body 32 outside of first lower hardness zone 78and second lower hardness zone 80. Extending throughout means that thesubject higher hardness zone 82 is pervasive in elongate link body 32apart from lower hardness zones 78 and 80, such that substantially all,such as at least 80-90% of elongate link body 32 except those parts thatare within lower hardness zones 78 and 80, is part of higher hardnesszone 82. Elongate link body 32 further includes a third inner surface 60and a fourth inner surface 62 forming a first bolting window 64 and asecond bolting window 66, respectively. Bolting window 64 and boltingwindow 66 are located longitudinally between first track pin bore 48 andsecond track pin bore 54. A strut 68 extends between first boltingwindow 64 and second bolting window 66. First lower hardness zone 78 mayinclude a portion of third inner surface 60, and second lower hardnesszone 80 may include a portion of fourth inner surface 62. Higherhardness zone 82 may include strut 68.

As discussed above, engagement and disengagement of rotatingtrack-contacting components in a track system that engage and disengagetrack links 20, in particular rail surfaces, can produce a wearphenomenon known in the art as scalloping. It has been observed thatcertain techniques such as providing hardened material in the areas ofsliding contact, providing extra, sacrificial wear material at selectedlocations, and still other strategies, can reduce the tendency forscalloping to occur and thus extend track service life. It can be seenfrom FIG. 3 that an entirety of upper rail surface 44 is included inhigher hardness zone 82. At least a majority of upper rail surface 44 iswithin higher harness zone 82 in most embodiments. Accordingly, whentrack link 20 is first placed in service all of upper rail surface 44can include relatively hard and wear-resistant material. Over the courseof service life of track link 20, however, even the relatively hardmaterial initially forming upper rail surface 44 will begin to wearaway. It can be noted from FIG. 3 that first lower hardness zone 78 andsecond lower hardness zone 80 are beneath higher hardness zone 82 alonga full extent of upper rail surface 44. As the relatively hard materialof higher hardness zone 82 wears away, eventually the softer material oflower hardness zones 78 and 80 will be exposed. What may be observed isan initial relatively mild scalloping phenomenon as wear of upper railsurface 44 initially proceeds according to a conventional pattern,concentrating material of wear toward the middle of elongate link body32. Once the relatively higher hardness material of higher hardness zone82 wears away to expose the softer material of lower hardness zones 78and 80, the less wear resistant softer material will begin to besacrificed at a relatively faster rate, hastening wear adjacent to orupon link straps 36 and 40 in a manner that compensates for thescalloping that tends to occur more in a middle portion of upper railsurface 44.

Referring also now to FIG. 5, there is shown track link 20 where somewear in the nature of scalloping has occurred, such that a relativelyshallow scallop 75 is formed generally at or about a longitudinalmidpoint of upper rail surface 44. It will be recalled that the tendencyfor an idler to slide into and out of contact with track link 20 cancause wear to be biased toward the middle part of the track link 20,between the opposite link ends, approximately as shown in FIG. 5.

Referring also to FIG. 6, there is shown track link 20 where elongatelink body 32 has worn further, such that the relatively softer materialof lower hardness zones 78 and 80 has been exposed and wear in thoseareas has accelerated. As a result, upper rail surface 44 has returnedto a more or less flat state, approximately as it appears prior to beingplaced in service, since the greater wear rate toward ends of the linkhas allowed wear to catch up in those regions. It should be appreciatedthat the illustrations in FIG. 5 and FIG. 6 are illustrative only and donot necessarily reflect an actual wear pattern that might be observed.In any event, as wear continues past a desired state, eventually theassociated machine will be idled for service and track 12 along withpotentially other undercarriage system components will be replaced. Ithas been observed that scalloping of track links 20 is a factor limitingservice life not only because material can eventually be worn awaybeyond the point at which acceptable operation and performance can beachieved, but also due to decreased operator comfort. In some instances,scalloping can become sufficiently severe that operators can no longercomfortably operate the associated machine. It is expected that applyingthe teachings of the present disclosure to existing track systems canimprove service life of a track more than 20%, and potentially more than30% more than what can otherwise be expected due to improvements inoperator comfort.

INDUSTRIAL APPLICABILITY

Referring to the drawings generally, but now in particular to FIG. 4,there are shown stages in methodology for making a track link 20according to the present disclosure. Making track link 20 can includeheat treating elongate link body 32 such that material, extendingthroughout elongate link body 32, is hardened. Material of whichelongate link body 32 is made can include an iron or a steel, shown byway of reference numeral 76. In FIG. 4 an initial heat stage 200 caninclude placing elongate link body 32 within a furnace or the like 100for a period of time sufficient to heat elongate link body 32substantially uniformly. Heat treats temperatures and process conditionsto achieve hardness and other material properties suitable forapplication in the present context are routine. From heat stage 200processing can advance to a quench stage 202 where elongate link body 32is liquid quenched. Heat treating elongate link body 32 by way of heatstage 200 and quench stage 202 can include hardening material 76extending throughout elongate link body 32 to a hardness greater than 50HRC (Rockwell C hardness scale). Link body material 76 can have ahardness after quench stage 202 of about 55 HRC or greater. From quenchstage 202 processing can advance to a temper stage 204 where a temperingapparatus 104 is employed to position heating elements 106, such asinductive heating elements, within first track pin bore 48 and secondtrack pin bore 52. At temper stage 204 tempering of elongate link body32 is performed such that a portion of link body material 76 that ishardened from heat stage 200 and quench stage 202, and forms first andsecond track pin bores 48 and 52, is softened. Tempering apparatus 104and specifications suitable for application in the present context areroutine. After tempering elongate link body 32 to soften link bodymaterial 76 that forms track pin bores 48 and 52, processing of tracklink 20 can advance to final machining. Tempering of elongate link body32 can include softening the subject link body material 76 to a hardnessless than 50 HRC. The softening can further include softening thesubject link body material 76 to a hardness of about 45 HRC or less. Theprocessing stages depicted in FIG. 4 enable forming, by way of the heattreating and tempering of elongate link body 32, the scallop-retardingpattern of varying hardness. Accordingly, when temper stage 204 iscompleted, link body material 76 may have a hardness greater than 50 HRCwithin higher hardness zone 82, and potentially about 55 HRC or greater.Link body material 76 may have a hardness less than 50 HRC in each offirst lower hardness zone 78 and second lower hardness zone 80, andpotentially about 45 HRC or less within each of lower hardness zones 78and 80.

Certain earlier strategies employed a greater number of processingstages than that which can be employed in accordance with the presentdisclosure. For instance, it was previously known to heat treat linkbodies to harden them, followed by softening the entirety of the linkbody, and then selectively rail hardening the upper rail surface 44. Itwill be appreciated that the present disclosure can provide anefficiency gain over such prior strategies given that only oneheat-treating procedure and one tempering procedure can be employed toproduce a piece suitable for final machining.

Certain other features made possible in part by the present disclosureare also evident from the depiction of elongate link body 32 at temperstage 204 in FIG. 4. In particular, it can be seen that higher hardnesszone 82, as shown by the consistent sectioning, includes at least aportion of lower shoe-mounting surface 42. It can also be noted thatelongate link body 32 has an outer perimetric surface 58 that forms aboundary. Outer perimetric surface 58 includes upper rail surface 44 andlower shoe-mounting surface 42. First lower hardness zone 78 and secondlower hardness zone 80 together form a negative image of higher hardnesszone 82 within the boundary formed by outer perimetric surface 58. Itcan still further be noted that a greater depth 74 of higher hardnesszone 82 is formed from upper rail surface 44 to lower shoe-mountingsurface 42, at a longitudinal location between first and second trackpin bores 48 and 52. A lesser depth 72 of higher hardness zone 82 isformed and extends from upper rail surface 44 to first and second lowerhardness zones 78 and 80 at longitudinal locations of first and secondtrack pin bores 48 and 52. Depth 72 may be about 20 millimeters or less.As used herein, the term “about” can be understood in the context ofconventional rounding to a consistent number of significant digits.Accordingly, “about 20” means from 15 to 24.5, and so on. It will stillfurther be noted that an arcuate interface 70 is formed by each of firstlower hardness zone 78 and second lower hardness zone 80 with higherhardness zone 82. Arcuate interfaces 70, which in the illustratedembodiment are discontinuous and separated by higher hardness zone 82,can be understood to include peaks (not numbered) positionedsubstantially at the same longitudinal locations as the axes 50 and 56of track pin bores 48 and 54. Such peaks may be also be understood aspeaks of first and second lower hardness zones 78 and 80. Depth 72 maybe measured, in a new link not yet placed in service, from upper railsurface 44 to the peaks.

Arcuate interfaces 70 can be circular, or partially circular, andgenerally extend circumferentially around axes 50 and 56 at constantradial distances. In other instances, tempering of elongate link body 32could include driving heat into the material being tempered at a varyingradial distance. In other words, rather than a generally circular orsemicircular shaped boundary (arcuate interfaces 70) of lower hardnesszones 78 and 82, in other instances a non-circular boundary might beformed by manipulating the inductive heating (or other heating process)by way of known techniques. It can also be appreciated that depth 72 canbe made greater or less depending upon the manner in which wear isdesired to progress. Track links 20 could be designed with the shapeand/or depth of the relatively harder link body material 76 overlayingthe relatively softer link body material 76 in a manner that is tailoredfor certain environments, operating conditions, machine applications,and substrate materials.

The present description is for illustrative purposes only, and shouldnot be construed to narrow the breadth of the present disclosure in anyway. Thus, those skilled in the art will appreciate that variousmodifications might be made to the presently disclosed embodimentswithout departing from the full and fair scope and spirit of the presentdisclosure. Other aspects, features and advantages will be apparent uponan examination of the attached drawings and appended claims. As usedherein, the articles “a” and “an” are intended to include one or moreitems, and may be used interchangeably with “one or more.” Where onlyone item is intended, the term “one” or similar language is used. Also,as used herein, the terms “has,” “have,” “having,” or the like areintended to be open-ended terms. Further, the phrase “based on” isintended to mean “based, at least in part, on” unless explicitly statedotherwise.

What is claimed is:
 1. A track for a ground-engaging machine comprising: a track chain assembly including a first track chain, and a second track chain extending in parallel with the first track chain, and each of the first track chain and the second track chain being formed by a plurality of track links; each of the plurality of track links forming the first track chain and the plurality of track links forming the second track chain including an elongate link body having a first link body end with a first link strap and a second link body end with a second link strap; each elongate link body including a lower shoe-mounting surface, an upper rail surface, a first inner surface defining a first track pin bore adjacent to the first link body end, and a second inner surface defining a second track pin bore adjacent to the second link body end; each elongate link body being formed of a link body material that varies in hardness within the elongate link body to form a first lower hardness zone, a second lower hardness zone, and a higher hardness zone; and the first lower hardness zone including the first inner surface and extending circumferentially around the first track pin bore, the second lower hardness zone including the second inner surface and extending circumferentially around the second track pin bore, and the higher hardness zone including at least a majority of the upper rail surface and extending throughout the elongate link body outside of the first lower hardness zone and the second lower hardness zone.
 2. The track of claim 1 wherein the higher hardness zone includes at least a portion of the lower shoe-mounting surface.
 3. The track of claim 2 wherein each elongate link body has an outer perimetric surface that includes the upper rail surface and the lower shoe-mounting surface, and wherein the first lower hardness zone and the second lower hardness zone together form a negative image of the higher hardness zone within a boundary formed by the outer perimetric surface.
 4. The track of claim 2 wherein each elongate link body further includes a third inner surface and a fourth inner surface forming a first bolting window and a second bolting window, respectively, located longitudinally between the first track pin bore and the second track pin bore, and a strut extending between the first bolting window and the second bolting window.
 5. The track of claim 4 wherein the first lower hardness zone includes a portion of the third inner surface, and the second lower hardness zone includes a portion of the fourth inner surface.
 6. The track of claim 5 wherein the higher hardness zone includes the strut.
 7. The track of claim 1 wherein the upper rail surface is flat.
 8. The track of claim 7 wherein each of the first lower hardness zone and the second lower hardness zone forms an arcuate interface with the higher hardness zone.
 9. The track of claim 8 wherein the higher hardness zone has a depth extending downward from the upper rail surface to peaks of the first lower hardness zone and the second lower hardness zone, and the depth is about 20 millimeters or less.
 10. A track link for a track in a ground-engaging machine comprising: an elongate link body including a first link body end and a second link body end, and having a lower shoe-mounting surface, an upper rail surface, a first inner surface defining a first track pin bore adjacent to the first link body end, and a second inner surface defining a second track pin bore adjacent to the second link body end; the elongate link body being formed of a link body material that varies in hardness within the elongate link body to form a first lower hardness zone, a second lower hardness zone, and a higher hardness zone; the first lower hardness zone including the first inner surface and extending circumferentially around the first track pin bore, the second lower hardness zone including the second inner surface and extending circumferentially around the second track pin bore; and the higher hardness zone including at least a majority of the upper rail surface and extending throughout the elongate link body outside of the first lower hardness zone and the second lower hardness zone.
 11. The track link of claim 10 wherein each elongate link body has an outer perimetric surface that includes the upper rail surface and the lower shoe-mounting surface, and wherein the first lower hardness zone and the second lower hardness zone together form a negative image of the higher hardness zone within a boundary formed by the outer perimetric surface.
 12. The track link of claim 10 wherein the upper rail surface is flat, and each of the first lower hardness zone and the second lower hardness zone forms an arcuate interface with the higher hardness zone.
 13. The track link of claim 12 wherein the link body material has a hardness greater than 50 HRC within the higher hardness zone and a hardness less than 50 HRC in each of the first lower hardness zone and the second lower hardness zone.
 14. The track link of claim 13 wherein the link body material has a hardness of about 55 HRC or greater within the higher hardness zone and a hardness of about 45 HRC or less within each of the first lower hardness zone and the second lower hardness zone.
 15. The track link of claim 10 wherein the higher hardness zone extends from the upper rail surface to the lower shoe-mounting surface, and wherein a depth of the higher hardness zone from the upper rail surface to peaks of each of the first lower hardness zone and the second lower hardness zone is about 20 millimeters or less.
 16. The track link of claim 10 wherein: the elongate link body further includes a third inner surface and a fourth inner surface forming a first bolting window and a second bolting window, respectively, located longitudinally between the first track pin bore and the second track pin bore, and a strut extending between the first bolting window and the second bolting window; the first lower hardness zone includes a portion of the third inner surface, and the second lower hardness zone includes a portion of the fourth inner surface; and the higher hardness zone includes the strut.
 17. A method of making a track link comprising: heat treating an elongate link body having a lower shoe-mounting surface and an upper rail surface such that a material extending throughout the elongate link body is hardened; tempering the elongate link body such that a portion of the material that is hardened and forms a first and a second track pin bore in the elongate link body is softened; and forming, by way of the heat treating and the tempering of the elongate link body, a scallop-retarding pattern of varying hardness where the elongate link body has a first lower hardness zone extending circumferentially around a first track pin bore, a second lower hardness zone extending circumferentially around a second track pin bore, and a higher hardness zone that includes the upper rail surface.
 18. The method of claim 17 wherein the heat treating of the elongate link body includes hardening the material extending throughout the elongate link body to a hardness greater than 50 HRC, and the tempering of the elongate link body includes softening the material that is hardened to a hardness less than 50 HRC.
 19. The method of claim 18 wherein the forming of the scallop-retarding pattern of varying hardness further includes: forming a greater depth of the higher hardness zone from the upper rail surface to the lower shoe-mounting surface at a longitudinal location between the first and the second track pin bores; and forming a lesser depth of the higher hardness zone that extends from the upper rail surface to the first and the second lower hardness zones at longitudinal locations of the first and the second track pin bores, respectively.
 20. The method of claim 19 wherein the lesser depth is about 20 millimeters or less. 